Many different types of digitally controlled printing systems of inkjet printing apparatus are presently being used. These inkjet printers use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. For home applications, digital inkjet printing apparatus is the printing system of choice because low hardware cost make the printer affordable to everyone. Another application for digital inkjet printing uses large format printers. It is a further requirement that these large format printers provide low cost copies with an ever improving quality. Inkjet printing technology is the first choice in today's art. Thus, there is a need for improved ways to make digitally controlled graphic arts media, such as billboards, large displays, and home photos for example, so that quality color images may be made at a high-speed and low cost, using standard or special paper.
Inkjet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because of its nonimpact, low-noise characteristics, its use of papers from plain paper to specialized high gloss papers and its avoidance of toner transfers and fixing. Inkjet printing mechanisms can be categorized as either continuous inkjet or droplet on demand inkjet. Continuous inkjet printing dates back to at least 1929. See U.S. Pat. No. 1,941,001 to Hansell.
U.S. Pat. No. 3,373,437, issued to Sweet et al. in 1967, discloses an array of continuous inkjet orifices wherein ink droplets to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous inkjet, and is used by several manufacturers, including Elmjet and Scitex.
U.S. Pat. No. 3,416,153, issued to Hertz et al. in 1966, discloses a method of achieving variable optical density of printed spots in continuous inkjet printing using the electrostatic dispersion of a charged droplet stream to modulate the number of droplets which pass through a small orifice. This technique is used in inkjet printers manufactured by Iris.
U.S. Pat. No. 3,878,519, h issued to Eaton in 1974, discloses a method and apparatus for synchronizing droplet formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.
U.S. Pat. No. 4,346,387, issued to Hertz in 1982 discloses a method and apparatus for controlling the electric charge on droplets formed by the breaking up of a pressurized liquid stream at a droplet formation point located within the electric field having an electric potential gradient. Droplet formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the droplets at the point of their formation. In addition to charging tunnels, deflection plates are used to actually deflect droplets.
Conventional continuous inkjet utilizes electrostatic charging tunnels that are placed close to the point where the droplets are formed in a stream. In this manner individual droplets may be charged. The charged droplets may be deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes referred to as a "catcher") may be used to intercept the charged droplets, while the uncharged droplets are free to strike the recording medium. If there is no electric field present or if the break off point from the droplet is sufficiently far from the electric field (even if a portion of the stream before droplets break off is in the presence of an electric field), then charging will not occur.
The on demand type inkjet printers are covered by hundreds of patents and describe two techniques for droplet formation. At every orifice, (about 30 to 200 are used for a consumer type printer) a pressurization actuator is used to produce the inkjet droplet. The two types of actuators are heat and piezo materials. The heater at a convenient location heats ink and a quantity will phase change into a gaseous steam bubble and raise the internal ink pressure sufficiently for an ink droplet to be expelled to a suitable receiver. The piezo ink actuator incorporates a piezo material. It is said to possess piezo electric properties if an electric charge is produced when a mechanical stress is applied. This is commonly referred to as the "generator effect". "The converse also holds true; an applied electric field will produce a mechanical stress in the material. This is commonly referred to as the "motor effect". Some naturally occurring materials possessing this characteristics are: quartz and tourmaline. Some artificially produced piezoelectric crystals are: Rochelle salt, ammonium dihydrogen phosphate (ADP) and lithium sulphate (LH). The class of materials used for piezo actuators in an inkjet print head possessing those properties includes polarized piezoelectric ceramics. They are typically referred to as ferroelectric materials. In contrast to the naturally occurring piezoelectric crystals, ferroelectric ceramics are of the "polycrystalline" structure. The most commonly produced piezoelectric ceramics are: lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate. For the inkjet print head a ferroelectric ceramic is machined to produce ink chambers. The chamber is water proofed by gold plating and becomes a conductor to apply the charge and cause the piezo "motor effect". This "motor effect" causes the ink cavity to shrink, raise the internal pressure, and generate an ink droplet.
Inks for high speed jet droplet printers must have a number of special characteristics. Typically, water-based inks have been used because of their conductivity and viscosity range. Thus, for use in a jet droplet printer the ink must be electrically conductive, having a resistivity below about 5000 ohm-cm and preferably below about 500 ohm-cm. For good flow through small orifices water-based inks generally have a viscosity in the range between about 1 to 15 centipoise at 25 degree C.
Over and above this, the ink must be stable over a long period of time, compatible with the materials comprising the orifice plate and ink manifold, free of living organisms, and functional after printing. The required functional characteristics after printing are: smear resistance after printing, fast drying on paper, and waterproof when dry. Examples of different types of water-based jet droplet printing inks are found in U.S. Pat. Nos. 3,903,034; 3,889,269; 3,870,528; 3,846,141; 3,776,642; and 3,705,043.
The ink also has to incorporate a nondrying characteristic in the jet cavity so that the drying of ink in the cavity is hindered or slowed to such a degree that through occasional spitting of ink droplets the cavities can be kept open. The addition of glycol will facilitate the free flow of ink through the inkjet. Inkjet printing apparatus typically includes an inkjet print head that is exposed to the various environment where inkjet printing is utilized. The orifices are exposed to all kinds of air born particles. Particulate debris accumulates on the surfaces, forming around the orifices. The ink will combine with such particulate debris to form an interference burr to block the orifice or cause through an altered surface wetting to inhibit a proper formation of the ink droplet. That particulate debris has to be cleaned from the orifice to restore proper droplet formation. This cleaning commonly is achieved by wiping, spraying, vacuum suction, and/or spitting of ink through the orifice. The wiping is the most common application.
Inks used in inkjet printers can be said to have the following problems:
1) they require a large amount of energy to dry after printing; PA1 2) large printed areas on paper usually cockle because of the amount of water present; PA1 3) the printed images are sensitive to wet and dry rub; PA1 4) the compositions of the ink usually require an anti-bacterial preservative to minimize the growth of bacteria in the ink; PA1 5) the inks tend to dry out in and around the orifices resulting in clogging; PA1 6) the wiping of the orifice plate causes wear on plate and wiper; PA1 7) the wiper itself generates particles that clog the orifice; PA1 8) cleaning cycles are time consuming and slow the productivity of inkjet printers. It is especially of concern in large format printers where frequent cleaning cycles interrupt the printing of an image; and PA1 9) when a special printing pattern is initiated to compensate for plugged or badly performing orifices, the printing rate declines. PA1 a) means defining an ink cleaning cavity spaced from the printing position for receiving cleaning fluid; PA1 b) an adjustable vane partially submerged in the cleaning fluid and spaced from the structure; and PA1 c) pumping means for delivering the cleaning liquid at a desired velocity towards the adjustable vane so that the vane creates a flow of cleaning liquid past the structure in the cleaning cavity space; and PA1 d) means for positioning the vane so as to create a desired flow of cleaning liquid which engages the structure such that the orifices are cleaned by the cleaning liquid.
Some of these problems may be overcome by the use of polar, conductive organic solvent based ink formulations. However, the use of non-polar organic solvents is generally precluded by their lack of electrical conductivity. The addition of solvent soluble salts can make such inks conductive, but such salts are often toxic, corrosive, and unstable.